Several local hams have been having problems with EFHW antennas going high on the SWR while on ft8 or cw and shutting down the rig’s transmit. These problems usually manifest on the 100 or 200 watt rated transformers when operated on digital modes on 30, 17, or 12. It can also be a problem on cw.
This applies to both 4010 and 8010s.
See the following:
On a typical EFHW installation on 30, 17, and 12 meters the SWR is moderately high but easily tuneable with most rig tuners.
The problem with that is that if you see 3 or 4 to one at the rig end of your coax (tuner off) it is likely the SWR is higher at the transformer. High SWR at the transformer box causes ferrite core saturation of the 49:1 transformer inside the box, which causes it to overheat and makes the impedance of the EFHW antenna wacky. Local hams have experienced rf in shacks, rig shutdown, etc.
The solution is to run low power on the bands where problems happen or get a bigger transformer. Personally, I have no amp but use the 1 KW transformer from Balun Designs. The extra cost is well worth not having to worry about saturating the core when I am on 30, 17, or 12 meters on ft8.
If I was running power I would get the highest-rated box I could find.
Many new hams, and some old ones, either don’t know or forget that an antenna tuner doesn’t really tune your antenna. It matches the 50 ohm output of your rig to whatever impedance presents itself at the rig end of your coax. That changes from band to band and even within bands. So using a tuner, either in your rig or an external one, makes it easy to use EFHWs on bands they aren’t harmonically related to. But you also must reduce the power by a factor of three or four.
My cheat for this is to reduce by the same integer as the SWR. So if you are seeing a 4 to 1 SWR at the rig end without your tuner, derate the transformer’s power capacity by dividing by 4. Remember this is not necessary on SSB or anywhere the system SWR is low (without your tuner inline.). For the typical 200 watt rated transformer (SSB rating) this means 50 watts on 30, 17, and 12. But don’t take my word for it, check your own installation first!
Using a 1000 watt rated transformer means not having to worry about exceeding the limits on those bands. The 1000 watt boxes don’t cost that much more than the 200 watt ones.
There is one other thing that often messes up installations. The EFHW depends on the coax shield to be the missing side of the antenna. You need enough coax between the antenna and your rig or tuner. The rule of thumb I see online is to use at least 35 feet. If you experience high local noise a choke at the rig end may help. I use an ugly balun outside the shack window at the coax pass-through and a second toroidal one inside. Using both has really helped my local noise situation.
Whatever you do, don’t put a choke right at the EFHW transformer. The antenna needs something to work against to radiate effectively. Your coax shield is that something. Most boxes have a counterpoise lug but I have found using a counterpoise to be problematic, as the length of the counterpoise can throw off normal tuning on one or more bands. My installation had a high SWR on 10 meters and would not tune 17 meters until I removed the counterpoise wire I thought I needed.
FWIW de NY4D
2 Thoughts to “End Fed Half Wave Antenna Problems”
it is not just end-fed antennas with this “problem” my 40m Windom can be “tuned” just fine with my MFJ versatuner when operating at 80m. however running anything over 25 watts on FT8 causes the ferrite in the feed point transformer balun to overheat
Yes that would be an impossible swr for any ferrite to handle. NY4D